A gas turbine generally includes an inlet section, a compressor section, a combustion section, a turbine section and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section progressively increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The compressed working fluid and a fuel such as natural gas is mixed within the combustion section and burned in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases are routed from the combustion section into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may cause a shaft to rotate. The shaft may be connected to a generator to produce electricity.
Due to the continuous surge in natural gas demand, the supply of pipeline natural gas may, on occasion, become unable to satisfy the demand for natural gas fuel. As a result, gas turbine operators are constantly looking for suitable alternate fuels to burn within their gas turbines in place of the natural gas until the natural gas supply is restored. One example of a potential alternate fuel is ethane which is a known highly reactive fuel (HRF). With the introduction of hydraulic fracturing as a means to extract natural gas, a large surplus of ethane has materialized throughout the world.
As can be expected, there are various technical challenges associated with substituting one fuel such as ethane or other a HRF for another in a gas turbine combustor, particularly in combustors that are highly tuned over a narrow range of operating conditions based on the various fuel properties (i.e. fuel density, reactivity and wobbe index) of natural gas fuel.
Highly reactive fuels (HRFs) such as ethane typically have a higher heating value (HHV) and Wobbe Number (described below), than natural gas. HRFs may be diluted with an inert such as nitrogen to reduce the Wobbe Number, to that of the pipeline natural gas. However, this process increases the costs and may thus lower the competitiveness of using an HRF as a substitute fuel.
The incoming gas Wobbe Number (WN) and the Modified Wobbe Index (MWI) of the gas supplied to the turbine are particularly important fuel properties. The WN is defined as:
  WN  =      HHV          SG      Where:
HHV is the higher heating value of the gas fuel; and SG is the specific gravity of the gas fuel or gas fuel and steam mixture relative to air. The WN is used as an interchangeability index to permit gas fuels of various heating values to be utilized in the same combustion system without changing hardware. Temperature is not included in this equation for WN because gas is typically delivered at approximately ground temperature with little variation throughout the year.
The MWI is defined as:
  MWI  =      LHV                  (                  SGx          ⁡                      (                          460              +                              T                g                                      )                              where:
LHV is the lower heating value of the gas fuel or gas fuel and steam mixture and Tg is the gas fuel or gas fuel and steam mixture temperature in degrees Fahrenheit. MWI more accurately measures the energy delivered through a fuel nozzle at a given pressure ratio than WN. This distinction between MWI and WN becomes very important when gas fuel is heated before delivery to the gas turbine.
The sudden increase in variation of gas properties that result when switching between natural gas fuel and a highly reactive fuel such as ethane significantly affects the operability of the combustion system. Since it would be impractical to tune the combustion system to account for this variation, operation beyond the capability of the combustor could result, leading to increased combustion dynamics and operation outside of emissions compliance.
Therefore, there is a need for a system and method for reducing the HHV of an alternate HRF such as ethane. The system and method should permit adjustment of the MWI over a wide range without the need for significant temperature adjustment of the gas fuel. The system and method should provide a diluent for reducing the LHV and the resulting MWI. The system and method should not require an additional fuel separator and a fuel superheater. The system and method should not significantly increase the cost of delivered gas per unit of energy when compared to existing fuel delivery systems. The system should provide for purging either the natural gas or the HRF from the fuel system.